Diesel engines use a much leaner air-to-fuel ratio than gasoline engines. The larger amount of air in the intake gas promotes more complete fuel combustion and better fuel efficiency, and thus lower emissions of hydrocarbons and carbon monoxide than gasoline engines. However, with the higher pressures and temperatures in the diesel engine, nitrogen oxides emissions, which include nitrogen oxide (NO) and nitrogen dioxide (NO2), known collectively as NOx, tend to be higher because high combustion temperatures cause the oxygen and nitrogen in the intake air to combine.
To comply with increasingly stringent government mandates regarding NOx emissions, engine manufacturers have developed several NOx reduction approaches. One such approach is exhaust gas recirculation (EGR), in which a percentage of the exhaust gas is drawn or forced back into the intake and mixed with the fresh intake gas and fuel that enters the combustion chamber. Another approach is selective catalytic reduction (SCR). The SCR process reduces NOx to diatomic nitrogen (N2) and water (H2O) using a catalyst and anhydrous ammonia (NH3) or aqueous NH3, or a precursor that is convertible to NH3, such as urea.
In addition to NOx emissions, diesel engines also produce particulate matter (PM), or soot, which is produced in comparatively larger amounts than that of gasoline engines. PM is a complex emission that includes elemental carbon, heavy hydrocarbons derived from the fuel, lubricating oil, and hydrated sulfuric acid derived from the fuel sulfur. One approach for reducing or removing PM in diesel exhaust is a diesel particle filter (DPF). The DPF is designed to collect PM while simultaneously allowing exhaust gases to pass therethrough.
These example approaches as well as others may result in, or require, cylinder pressures that are relatively high, as compared to cylinder pressures in systems not using such approaches. These higher cylinder pressures create higher forces, and these higher forces are then applied to the intake and exhaust valves, the pistons, the connecting rods, and the crankshaft. Such forces may result in failures to these and other components in the engine.